Acoustically filtered phonographic reproducer



Patented Feb. 13, 1951 UNITED STATES PATENT OFFICE ACOUSTICALLY FILTERED PHONOGRAPHIC REPRODUCER Application September 27, 1946, Serial No. 699,675

1 Claim. 7 1

This invention relates to a phonographic reproducer of the acoustical type such as is commonly used on acoustical-type dictating machines; more particularly, the invention relates to providing such reproducers with an acoustical filter adapted to eliminate substantially all sound radiation to the outside air from the vibratile parts of the reproducer.

Acoustical-type reproducers include a sound box closed at the bottom by a vibratile diaphragm. This diaphragm is vibrated by a moving record through a stylus system so as to reproduce the recorded sounds, the reproduced sounds being collected by the sound box and being thence conveyed to a receiver such as a horn or headset. Such reproducers also radiate sound to the outside air, which radiated sound is commonly referred to in the art as needle talk. When the reproducers are used to reproduce high-level recordings such as are made, for example, by electrically operated phonographic recorders, the radiated sounds attain such high volume level as to obscure the intelligibility of the reproduced sounds from the horn or headset and to be very annoying to others who may work in the vicinity of the reproducing machine. I have found that this undesired sound radiation from the vibratile parts of an acoustical-type phonographic reproducer can be eliminated very simply by providing the reproducer with an appropriate acoustical filter and that this filter, when properly designed, will not aiiect deleteriously either the quality or level of response of the reproducer.

It is accordingly an object of my invention'to provide an improved acoustical-type phonographic reproducer having a high fidelity and eiliciency of sound reproduction which is substantially without undesired sound radiation or needle talk.

It is another object to provide an improvement for acoustical-type reproducers of dictating machines, which is adapted to increase the clearness and intelligibility of reproduced sounds so that they can be transcribed faster and with greater accuracy.

' It is another object to provide an acousticaltype phonographic reproducer with an acoustical filter which comprises a protective closure for th diaphragm of the sound box.

It is another object to support the stylus system of the reproducer by the protective closure of the acoustical filter.

It is a further object to fulfil the aforestated objectives by very' simple" apparatus which can be mounted on standard acoustica-l type phone graphic reproducers as heretofore manufactured and sold.

Other objects and features of my invention will be apparent from the following description and the appended claim.

In the description of my invention reference is had to the accompanying drawings, of which:

Figure 1 is a central vertical section through a phonographic reproducer embodying my invention;

Figure 2 is a bottom plan view of this reproducer; and

Figure 3 is a circuit of the approximate electrical analogue of the filter.

The phonographic reproducer shown in the accompanying figures comprises a basic reproducer arrangement common in the art such as is illustrated by the Holland Patent No. 1,207,404, patented December 5, 1946. Such a reproducer arrangement comprises a sound box I including a circular casing 2 having integral therewith a tubular neck 3 which is adapted to be connected to a horn or headset (not shown). This casing has an internal sound chamber 4 which has an opening leading through the neck 3 and which is closed at the bottom by a diaphragm 5. The diaphragm is mounted between two rubber gaskets 6 and I which are held in place by a ring 8 that is screw-threaded to the interior surface of an annular flange 9 of the casing, there being a metallic ring to interposed between the gasket I and the ring 8.

Below the sound box there is a depending arm H for supporting a stylus system that is adapted to couple operatively the diaphragm to a, moving phonographic record R fractionally shown, by wayof example, as of the cylindrical type. This arm has a forked bottom portion l2 which car-- ries a pivot pin I3 in parallel relation to the diaphragm. Straddling the forked portion are two arms M of a bracket I5, which have apertures engaged respectively by conical end portions of the pin l3. This bracket has two verticallyspaced lugs l6 and H. In the upper lug there is a hole forming a bearing I8 and in the lower lug there is a threaded hole engaged by, a screw l9 that has a V-bearing 20 at its inner end portion; the screw I 9 being held in adjusted position by a lock nut 2|. The bearings l8 and .20 are engaged by the conical ends 22 of a pivot shaft 23. To this shaft is secured a biasing weight 24 which extends diametrically from the bracket !5 across the sound box. It will be observed that this floating weight isuniversally supported in thatit has an up-and-down freedom of movement relative to the record about the pivot rod I3 and a lateral freedom of movement in the bearings l8 and 2B. Extending from the free end portion of the biasing weight is a pin 25 which passes through a U-shaped member 26 that is carried with the sound box so as to restrict the biasing weight to a limited range of movement.

The biasing weight is provded with a central hole 21 and with two depending standards 28 at the side of the hole 27 which is nearest the free end of the weight. These standards carry a pivot pin 29 on which is pivoted a stylus lever 30. This stylus lever is connected at its inner end to the diaphragm by a l nk 3| that passes through the central hole 21 of the biasing weight. At the free end of the stylus lever there is mounted a reproducing stylus 32 which is adapted to ride in the groove of a phonograph record. Normally, the phonographic reproducer is carried above the record and the stylus pressure on the record is provided. by the force of gravity acting on the weight 2Q. During reproducing, ready trackng of the stylus in the record groove is enabled by the universal freedom of movement afforded the biasing weight. It wil however be understood. that for the purposes of my invention the stylus pressure need. not be provided by the gravitational force on the weight 24 but may be aug mented or supplied substantially by a springbiasng of the member 24 provided by a torsional spring 33 on the pin l3 as illustrated in Figure 1. When the stylus pressure is produced chiefly by spring action, the reproducer may be operated in a vertical position as well as in the horizontal position shown. During operation of the reproducer the d a phragm is vibrated according to the modulations in the groove of the record to generate. corresponding sounds within the sound chamber 4. These sounds are then conveyed through the neck 13 to the horn. or headset. Also, the diaphragm will. generate sounds at its lower side which are radiated to the outside airin the reproducers as heretofore commonly used. Theoretically, the stylus lever and link 3| also radiate sounds but these are negligible in comparison to those radi ated by the diaphragm. In order to eliminate. the undesired sound radiation of the diaphragm, I; have mounted an acoustical filter 34 on the bottom of the sound box across the diaphragm. This filter is constructed so as to have an attenuation band through the audio spectrum of the frequency response range of the reproducer. This is-accompl'shed preferably by providing a filter of the low-pass type which has its cut-off. frequency at the low end that spectrum.

This filter 34 comprises a closure member 35 that is made preferably integral with the ring 8 aforementioned. lhis closure memberwhich serves also as a protective means for the diaphragm-is spaced from the diaphragm to form an internal sound chamber 353 at the side of the diaphragm opposite the chamber 4. Th s chamber. 33 is separate from the chamber 4 and is closed except for a restrictive orifice 31 to pass thelink 3|. However, the chamber has a sum-- cient volume so that it will not exertv any undesired restrictive influence on the free vibration of. the diaphragm. ihus the acoustical filter is prevented. from adversely affecting the response of the reproducer.

Preferably l1 Sl'tlJlQOll} the entire stylus system. of the reproducer by closure member 35 since this facilitates the. assembling and servicing of the neptoducer .To this endthe. arm I1 is se- 4 cured as by soldering to the closure member as indicated in Figure l.

The orifice 31 in the closure member has preferably a minimum diameter providing only the necessary clearance for the link 31. The orifice is however made eifectively long by an internal tube 38 in the chamber 36 which is secured at its outer end to the closure member. When the orifice is made effectively long and small in diameter the inertance of the air within the orifice is made large enough that with a volume of the chamber 35 provided readily in practice the resonant frequency of the chamberwhich is slightly below the cut-off frequency of the filter-occurs at the lower end of the frequency-response range of the reproducer. Thus the attenuation band of the filter is throughout substantially the entire response range of the reproducer. I have found that when the resonant frequency is of the order of 300 cycles per second the average attenuation in the useful frequency-response range of the reproducer is generally of the order of 15 decibels. This is a substantial attenuation which in practice is found to eliminate almost entirely all outside sound radiation or so-called needle talk.

The approximate electrical analogue of the acoustical filter'34 is shown in Figure 3. In this circuit the driver D corresponds to the diaphragm 5 and typically has an internal impedance which is high. relative to the impedance ofv thefilter. The capacity C corresponds to the acoustical capacitance of the chamber 36 and has they value where V is the volume of the chamber 36, d is the density of air and c is the velocity of sound. The inductance M corresponds to the inertance of the orifice 31 and has the value where L is the length of the orifice 31 in ems. and T is its radius in cms. The resistance Rm refers to the frictional resistance of the orifice 31 and has the value and the resistance RL refers to the radiation resistance of the orifice, this being the load for the filter, and has the value 2.2 10 f where f is the frequency in cycles per second. The resonant frequency f0 of the filter has then the value and the attenuation for thefilter is equal to the formula proximately .83 cm- With these values the resonant frequency of the chamber 36. is, approxir were we. e be stood that these values are illustrative and that therefore no unnecessary limitation thereto is intended since the values are not particularly critical.

It is to be noted that I may use as an additional element a suitable sound-absorption material 40 such as Celotex or felt within the sound chamber 36 as indicated in Figure 1. This soundabsorption material improves slightly the lowfrequency characteristic of the filter in the vicinity of its resonant frequency. However, it is not considered that the use of this sound-absorption element is important in most practical applications.

While I preferably employ, and have herein particularly shown and described, a simple onestage filter of the low-pass type for the purposes of my invention, it will be understood that in the broader aspects of my invention I may employ filters of two or more stages as well as other suitable types of filters and that I intend therefore no unnecessary limitation of my invention to the type of filter herein particularly described. It will moreover be apparent that other modifications and changes may be made without departure from the scope of my invention, which I endeavor to express according to the following claim.

I claim:

In an acoustical-type phonographic reproducer comprising a sound box having a vibratile diaphragm, a biasing weight supported at one side of said diaphragm for universal pivotal movement, a stylus lever pivoted to said biasing weight on an axis substantially parallel to said diaphragm, and a member coupling one end of said stylus lever to the central portion of said diaphragm: an acoustical filter on said sound box for attenuating the radiation of sound from said one side of the diaphragm to the outside air comprising a closure member secured at its edge in airtight relation to the rim of said diaphragm to provide an air chamber between the diaphragm and the closure member, and a single open-ended tubular member extending through the central portion of said closure member having a clearance opening leading from said chamber and through which passes said coupling member, said tubular member having a length sufficient to provide an air interance resonant with the compliance of the air in said air chamber at a frequency in the lower portion of the audio spectrum, and said. compliance being substantially greater than that of said diaphragm.

JOHN HORNBOSTEL.

REFERENCES CITED The following references are of record in the file of this patent:

' UNITED STATES PATENTS Number 

